Satellite Studies of Ship Tracks Show Complex Influence of Pollution on Clouds

SAN FRANCISCO – New satellite studies analyzing the impact on clouds from ocean-going ships suggest that airborne particles from pollution or other causes can have a drying effect on some clouds, and a saturating effect on others, complicating global climate change models.

The key difference is what kind of clouds become “perturbed” by the pollution and what the overlying atmospheric conditions may be, the researchers say.

“One significant impact is that our observations suggest clouds affected by haze will offer less reflectivity and less of a cooling effect than most global climate change models suggest,” said Jim Coakley, a professor in the College of Oceanic and Atmospheric Sciences at Oregon State University and principal investigator in the study.

Coakley presented the findings today at the annual meeting of the American Geophysical Union in San Francisco.

The ships are not necessarily a major cause of pollution, but the particles they emit into the air affect the overlying clouds and these effects can be easily tracked by NASA satellites. The impact of those emissions on the clouds is providing researchers with new information about cloud “behavior.”

Previous studies by Coakley and OSU graduate student Matt Segrin found that particles emitted by ships led to smaller water droplets and a general drying out of the clouds. Yet most models suggested just the opposite – that increasing the number of particles should lead to more liquid in the droplets.

“What we found was that most of those earlier satellite observations were taken off the coast of California and Oregon, where the marine cloud bank was heavy and the air above it comparatively dry,” Coakley said. “The clouds tend to suck down the drier air above them and because the droplets are smaller than normal from the pollution, they evaporate more readily. That evaporation cools the dry air which then sinks, drawing in more air from above, and the pollution clouds end up drying themselves out.

“In our latest studies, using NASA’s Aqua and Terra satellites, we followed the ships past the marine deck and out to where the nearby pristine clouds are dissipating,” Coakley said. “There the ships’ emissions actually create a cloud where clouds would not otherwise appear. We think that the atmosphere above these regions isn’t as dry so that the polluted clouds with their smaller droplets survive, while the nearby unpolluted clouds dissipate because their larger droplets and the support of the relatively moist air aloft grow to form drizzle and fall from the sky.”

When water molecules condense, they attach to a particle and make droplets. When there are few particles in the air, the droplets are bigger and more likely to reach the saturation point and fall as precipitation. Where there is pollution, more particles are in the air and the droplets become smaller, making it harder for them to grow to drizzle-sized droplets.

Because the pollution generates more particles and the water molecules adhere to them, many scientists have speculated that the clouds would become brighter and more reflective. And since burning fossil fuels would likely result in more particles, overall reflectivity would be higher. But the ship track research points out that isn’t necessarily the case. Those same droplets draw in the warmer air above and essentially evaporate, actually lowering the reflectivity to levels below that predicted by climate models.

“Cloud formation and response to pollution and environmental conditions is the weakest part of global climate change studies,” Coakley said. “What we are learning from our studies of polluted clouds is helping us better understand how all clouds behave.”

The researchers use near-infrared radiation to identify exactly where clouds have been polluted from the ships’ emissions then look at either side of the cloud to see the impact of that interaction on the nearby pristine clouds. The ships act as a kind of laboratory, creating tracks of pollution emissions easily visible via satellite imagery.

Ships are not alone in their impacts on clouds, Coakley pointed out, they are merely a convenient tool for research. Coal plants, automobiles and other pollution-causing agents also send particles into the air that can have an effect on cloud formation and behavior.

“You should see the pollution plumes from Los Angeles and San Francisco in the satellite images,” Coakley said. “They are so intense it’s difficult to follow the ship tracks in those locations.”

Coakley has been studying ship tracks and their impacts on clouds for more than 15 years and says the research provides data that should be helpful to climate modelers. It is premature to jump to too many conclusions about how clouds will react to global climate change without more years of research, he emphasized.

“These impacts from the ship tracks are local,” he said, “and we’re just discovering how important the air above the clouds is to their response. There may be many other factors that affect how particles form droplets, how the clouds respond, and what the result is in terms of reflectivity.”